1. Field of the Invention
The present invention relates to a method for the production of a fiber-reinforced hydraulically setting material, particularly a cement material, having two fibrous components, as well as to molded elements of all kinds produced from such materials.
2. Description of the Prior Art
Asbestos reinforced cement objects have proven themselves well and have assumed a firm place as construction materials during recent decades. Particularly wide-spread in the industry is the manufacture of a variety of structural components such as pipes, corrugated plates, roof shingles, etc., using de-watering procedures, for example as described by Magnani [see Heribert Hiendl, "Asbestzementmaschinen", page 42 (1964)] or Hatschek [see below]. One preferred method, the rolling method, as described by Hatschek, has been known for several decades [AT-PS 5970].
These known methods for the production of, for example asbestos cement pipes and plates, are based on the use of endless wet machines. In this type of machine a very thin asbestos suspension is coated over a chest and a revolving screen drum in the form of a non-woven fabric on a felt and is then rolled to the desired thickness with the aid of shaping rollers or pipe cores. Depending on the type of asbestos fiber used, the following problems may arise during this process:
The run-of-the-mine, pre-opened asbestos must be further opened in spreading apparatus in the asbestos cement factory, i.e., it must be further spread or opened in a edge mill. One of the most difficult problems is opening different natural types of asbestos fiber without shortening them and developing dust, if the degree of opening may not exceed a certain amount, because otherwise water removal or transport difficulties may occur on the endless wet machine.
In addition to opening the asbestos, a proper composition of the various types of asbestos fiber, for example length, talc content, etc., are of basic significance for proper machine operation and quality of the products to be manufactured.
The spreading of the asbestos and the mixing of the various types of asbestos have a decisive effect on production and quality of the final products. Only by controlling these parameters is it possible to obtain weatherproof products with good mechanical characteristics. The chest mold for the endless wet machine and the agitators also play an important role in the proper distribution of the asbestos fibers in the fleece with respect to the fiber direction of the asbestos in the final product. The fiber distribution in the fleece is significant for the economic use of the asbestos fibers, because with poor chest geometry and agitator effects there is the danger of asbestos concentrations in the fleece, whereby uniform fiber reinforcement in the product is impaired. Further, this type of asbestos concentration is disadvantageous with regard to the behaviour of the product in areas endangered by frost and with regard to the adhesion characteristics of color coatings.
The vacuum commonly used in various types of vacuum boxes must be correctly adjusted, while the water is being removed from the asbestos cement fleece on the felt-sieve, depending on the spread of the fibers. If this is not done cement particles may be torn out of the fleece or the fleece may not be sufficiently de-watered, which results in poor products during rolling.
During the rolling process additional water is generally removed from the resulting product by compression. The pressure applied must be adapted to the water content of the fleece as well as to the rolled wall thickness. If this is not done rigidity problems or loss of quality may occur.
In addition to such technical problems and settings in the production train necessary to ensure successful processing, these known procedures are based on the excellent affinity and the filtering action with respect to cement, i.e., the capacity of the asbestos fibers to retain cement. In addition to the good cement retaining action of the asbestos fibers, however, they also serve simultaneously as reinforcing fibers in the final product.
These two advantageous characteristics of asbestos fibers are accompanied by a very specific disadvantage. The natural physical characteristics, particularly the low breaking elongation of asbestos, gives pure asbestos cement products a certain brittleness. This characteristic is demonstrated by limited impact resistance. Continuous attempts have been made to find new cement reinforcing fibers which might lead to more flexible final products.
In a 1951 patent relating to the manufacture of asbestos cement products (German Pat. No. 878,918), the reinforcement of cement with fibrous materials such as cellulose or other organic or inorganic fibers is disclosed. During recent years a great number of natural and synthetic fibers have been tested in this context with regard to their suitability as cement reinforcing fibers. For example, tests have been performed with cotton, silk, wool, polyamide fibers, polyester fibers, polypropylene fibers and inorganic fibers such as glass fibers, steel fibers, carbon fibers, etc.
The construction material industry has published a number of methods for the production of wood reinforced cement products. Examples are: German Pat. Nos. 585,581; 654,433; 818,921; 915,317; British Pat. Nos. 252,906; 455,571; SE-OS 13139/68; SE-PS 60,225 and Swiss Pat. No. 216,902.
All methods disclosed in these patents, however, are concerned with the processing of cement with a minimum quantity of water, necessary for the hydraulic setting. The technology of mixing cement, wood chips and water, as well as the manufacture of construction materials from these mixtures, is completely different from the known Hatschek process, which operates with diluted aqueous slurries. Any preliminary treatment of the wood materials disclosed in the above patents with various mineral salts serves only to stabilize or mineralize the cellulose components of the wood, which swell when exposed to water. The mineral salts can also serve to deactivate deleterious materials in the wood which could impair setting of the cement, so that a good binding between wood and cement is assured.
In view of the technical difficulties described above, which can occur with the de-watering machines commonly used in the asbestos cement industry, it is apparent that a simple substitution of other fibers for asbestos fibers leaves it practically impossible to produce satisfactory fiber reinforced cement products on a large scale with the methods and apparatus commonly used. The proposed methods have also never been realized in industrial production.
One of the greatest problems with other than asbestos fibers has been the poor distribution of these fibers in the cement-water slurry. The fibers separate from the mixture and form knots. The poor cement retention capacity of most fibers also renders industrial production impossible. Furthermore, the strength added to the cement products by many synthetic fibers is a minimum, primarily because hydrophobic, organic fibers provide only poor adhesion in the cement matrix. It has been determined, however, that with an addition of a small quantity of asbestos, the production of fiber reinforced products according to current de-watering processes is possible (British Pat. No. 855,729). The addition of from 0,5 to 5% asbestos makes it possible for organic and inorganic fibers to be better distributed in a cement-water slurry, while simultaneously assuring sufficient cement retaining effect during the de-watering process.
To improve the adhesion of fibers in the cement matrix it has been proposed to use fibrillated polyamide films (U.S. Pat. No. 3,591,395).
In the U.S.S.R. magazine "Polim. Stroit. Mater.", 1975, 41, pages 5127, [C. A. 86, 7766/Z (1977)] it is stated that fibers with a rectangular cross section exhibit improved adhesive capacity. Further inventors describe thermoplastic fiber sections provided at their ends with a wider, melted portion, to improve the anchoring of these fibers in a cement matrix [Ja-AS 7,403,7404].
In German OS No. 2,819,794 it is proposed to produce fiber reinforced cement plates with the aid of specially modified polypropylene fibers of two different lengths. Where de-watering processes are used in manufacturing the mixture of the polypropylene fiber sections is premixed with cellulose fibers and with a portion of a cement-water slurry before the desired concentration of solid matter necessary for operation is adjusted. However, the process is limited to the use of specially modified polypropylene fibers with defined mixtures of different fiber lengths. Other fibers cannot be used for this purpose.
For various reasons, however, it is desirable to produce fiber reinforced cement products with good mechanical characteristics on production apparatus as presently used in the asbestos cement industry and to be able to produce these products without the aid of an admixture of asbestos and with the use of conventional fibers.